Distance-measuring interferometer

ABSTRACT

An interferometer for accurately measuring the displacement of a remotely positioned object includes means for projecting a first polarized beam of light at the object, for reflecting the projected beam from the object and for combining a reference beam of light with the reflected beam. The reference beam is of similar polarization and is orthogonally related to the first beam. Means are provided for converting the resultant beam to linearly polarized light and for detecting and indicating variations in the polarization plane of linearly polarized light. Displacement of the object causes variations in the plane of linear polarization and the magnitude, velocity, and direction of displacement are sensed and indicated.

United States Patent [72] inventor William Reid Smith-Vaniz 3,379,8874/1968 Stephany 350/149 14 Pasture Lane. Darien, Conn. 06820 OTHERREFERENCES 1 9T 3 "Elliptical Polarizer Block: .105. A. 1960 3; e 09".?rse 050 J l 18 1966 Messung Sehr Kleiner Verschiebungen mit einenPolariza- I msmn o a u) tionlnterferometer," Lebesque;Optik;Nov. 1964.

abandoned. [45] p d Aug, 2 1971 Primary Examiner-Ronald L. WibertAssistant ExaminerConrad Clark Altomey Edward R. Hyde, Jr.

[54] DISTANCE-MEASURING INTERFEROMETER 4 Claims 6 Drawing Figs. ABSTRACT: An lnterferometer for accurately measurlng the displacement ofa remotely posltloned ob ect includes means, (52] US. Cl 356/106 forprojecting a first polarized beam f light at the Object, for 1 Cl Golb9/02 reflecting the projected beam from the object and for combin- [50]Field ofSearcll 88/141, 1 m a reference beam f light with the fl d beamThe U; 356/1064 13; 350/149-151 reference beam is of similarpolarization and is orthogonally related to the first beam. Means areprovided for converting [56] References cued the resultant beam tolinearly polarized light and for detecting UNITED STATES PATENTS andindicating variations in the polarization plane of linearly 3,409,375 11/1968 Hubbard 356/106 polarized light. Displacement of the objectcauses variations in 3,127,465 3/1964 Stephens... 356/106 the plane oflinear polarization and the magnitude, velocity, 3,182,551 /1965 Piller88/39 and direction ofdisplacement are sensed and indicated.

10 MM BEAM il name REFLECWR L o f? 1.2"? in l lflStR I 6' l D 16 a m M lf 20 now/rm of 46 1/ I nes 1 1 I :4 l l2 :0 l g 3.9

T' AMPLIFIER 8+ o 8- cz/PPm J3 v i lam/Ream call/vim 8-- CL IPPER lPATENIED AUB24 I97! SHEET 1 OF 3 PATENTED AUG24 I97! SHEET 3 [1F 3 iDISTANCE-MEASURING INTERFEROMETER This application is acontinuation-in-part of US. Pat. application Ser. No. 566,050, filed onJuly 18, 1966 and now abandoned.

This invention relates to interferometers and more particularly tointerferometers adapted for measuring distance.

Interferometers which are employed for providing highly accuratemeasurements of distance generally comprise means for projecting a lightbeam from a source to an object whose distance is to be measured, forreflecting the beam from the object, and for interfering the projectedbeam with the reflected beam in order to establish a pattern ofinterference fringes. This class of interferometer is adjusted at timesfor providing that the zero order fringe fills the field. Fringe motionthen refers to alternate brightening and darkening of the entire field.A displacement in the position of the object causes a correspondingmotion of the fringes. Means also are provided for detecting andindicating the amount of fringe motion thereby providing an indicationof theobjects displacement.

lnterferometers of this type were generally limited to mea surements ofobject displacement over a relatively small distance because of thebandwidth or temporal incoherence of available light sources. However,the advent of the high intensity coherent light source such as providedby the laser renders it possible to now extend this measurement over arelatively large distance, i.e., feet.

It is an object of the present invention to provide an improved form ofinterferometer adapted for detecting small displacements of an objectover a relatively large distance.

Another object of the present invention is to provide an improved fonnof interferometer adapted for detecting and measuring displacements ofan object over a relatively large distance.

In addition to indicating the magnitude of object displacement, it isalso desirableto indicate the direction of displacement. Arrangementsfor counting moving fringes have em-' ployed a photocell to sense fringemotion and to indicate the magnitude thereof. However, additional meansare required to sense and indicate the direction of motion. One form ofinterferometer employs two photocells and beam splitters which It isfurther desirable to sense and indicate the velocity of displacement ofthe object.

It is another object of this invention to provide an improvedinterferometer for indicating the magnitude and direction ofdisplacement of an object over a relatively large distance.

Another object of the invention is to provide means for-indicating thevelocity of displacement of the object.

In accordance with'a feature of the present invention, an interferometerfor measuring the displacement of an object at a relatively largedistance includes means for projecting at the object a polarized beam oflight, means for reflecting the polarized light beam from the object andfor combining with the reflected beam a reference beam which issimilarly polarized and orthogonally related with respect to thereflected beam. Means are provided for converting the resulting beaminto linearly polarized light. Movements of the object cause anaccompanying rotation in the plane of polarization of the resultantlinearly polarized light and a corresponding movement of fringesproduced by the interference. Means are provided for detectingvariations in the plane of the linear polarization and for indicatingdisplacement of the object.

In accordance with another feature of the invention, means including asingle photocell are provided for detecting variations in the plane oflinear polarization and for indicating the velocity and the direction ofdisplacement of the object.

single photocell for detecting the velocity of displacement of theobject; and,

FIG. 5 is a diagram illustrates a further embodiment of the invention.

The general aspects of the present invention can best be understood byconsidering polarization'of light as represented by points on a Poincaresphere. The use of the Poincare sphere is a technique for analyzing theresult of the combination of light of differing polarizations. It isdescribed in Polarized Light, Production ,and Use, William A. Shurcliff,Harvard University Press, Cambridge, Massachusetts, 1966, pp. 15-19, pp.95-99. Orthogonally polarized light beams, which are discussed at page 6of the referred-to text, combine to form a light beam defined by acircle on Poincares sphere. The combined polarized beams compriseorthogonally related linearly polarized light, orthogonally relatedcircularly polarized light (FIG. I), andorthogonally relatedelliptically polarized light (FIG. 5). Orthogonally related beams cancomprise two forms of linear polarization that differ by 90 in azimuthwhen the directionof propagation is the same. Right and left circularlypolarized beams are orthogonal. Two elliptically polarized beams areorthogonal when the azimuths of the major axes differ by thehandednesses are opposite, and the elliptici ties are the same. Thecircle representingthe resultant of the clude a source of high intensitycoherent light comprising a laser 10. A light beam output from the laseris directed at a retroreflector 14 via a collimator including lensl6-and 18, a

one quarter valve plate 20 which transforms the laser beam tocircularly-polarized light, and a first beam splitter 22. The beamsplitter 22 comprises an optical quality glass having a half-silveredsurface 24 for providing'50 percent transmission, and 50percentreflection. A circularly polarized beam of light 1,, indicated as25 and which is-incident on the surface 24 is divided by the beamsplitter into a transmitted component l t, and a reflected component l rThe factors I, and r respectively represent the fraction of I, which istransmitted and reflected. The sense of circular polarization of I,,twhich is projected-to the reflector 14 is indicated to be in a clockwise.direction while the reflection at surface 24 causes l,,r to bepolarized in a counterclockwise direction.

Retroreflector 14 is a conventional reflector means which is coupled toan object 26 (discussed hereinafter) for-movement with the object. It isadapted to provide an odd number of reflections of l t thus reversingthe sense of circular polarization of the beam. The reflected beam l t,is indicated to be circularly polarized in a counterclockwise direction.The retroreflector comprises a corner cube with metallic reflectivesurfaces. As indicated, the number of reflections is odd, providing forthe reversal in the direction of polarization. In

addition, the use of a corner cube provides insensitivity to mountingangle.

A second beam splitter 27 is positioned with respect to the reflector 14and beam splitter 22 for providing impingement on a surface 28 thereofby the reflected beam I,,t and the beam I,,r,. These beams at surface 28are circularly polarized in an opposite sense and, neglecting any smallloss in the transmission to and reflection from reflector 14, are of anequal intensity A,. They thus produce a resultant linearly polarizedwave I having an angle of polarization 0, measured with respect to somereference axis, and which varies in accordance with the distancetraversed by the transmitted beam I,,t,. Beam splitter 27 is formed ofan optical quality glass and includes a surface 28 which ishalf-silvered for providing 50 percent transmission and 50 percentreflection. Linearly polarized resultants of the combined beams comprisethe beams I,, and I' where I I,,( r,t +t r and I' ,,(r,r +t,t The beam1', impinges upon a monitor 29 which is provided for viewing fringes.The beam 1, impinges upon a third beam splitter 30 of a detector andindicator means which is indicated generally as 31.

Included in the detector and indicator means 31 are means fortransmitting a linearly polarized interference pattern at differentangles of polarization and for providing electrical output signalsindicative of the magnitude and direction of the fringe movement. Thebeam splitter 30 comprises an optical quality glass having ahalf-silvered surface 33 adapted for providing 50 percent transmissionand 50 percent reflection. Output beams from the splitter 30 aretransmitted along a preferential axis of polarization through polarizers32 and 34 and impinge upon photocells 36 and 38 respectively. Thepolarizer 32 is adapted to transmit linearly polarized light polarizedat a first angle with respect to some reference axis while the polarizer34 is adapted to transmit polarized light at a second phase anglediffering from 0. For example, the second polarization angle may equal0+45. Thus, at a constant velocity of the retroreflector, an outputsignal e from photocell 38 will differ in phase with an output signal efrom photocell 36 by 90. These signals are amplified and clipped byconventional circuit means, represented by the blocks 40 and 42. Whenthe object 26 is displaced, the output from the circuits 40 and 42comprises two electrical signals having a relative phase which isindicative of the direction of displacement. These signals are appliedto a bidirectional counter 44 which is adapted to increase anaccumulated count when the object displacement is in one direction andto reduce the count when the object displacement reverses to an oppositedirection. A counter of this type is described in U.S. Pat. No.3,287,544.

In FIG. I the object 26 whose displacement is to be measured is shown tocomprise a movable transport bed 26 of a machine tool (not illustrated).In order to illustrate the optical arrangement in greater detail, therelative size of the reflector 14 with respect to the bed 26 is greatlyexaggerated. The bed may be driven by suitable means through ring gears49, along a rack 46 to thereby provide motion of the tool. In anautomated machining operation, indications of small variations in themotion of the bed and thus the tool are to be detected. As the bed 26 ismoved from an initial position, the retroreflector 14 is similarlydisplaced. The plane of polarization of the linearly polarized waveproduced by an interference of the beams rotates as the retroreflector14 is displaced. The plane of polarization will rotate 360 for eachdisplacement (d) which is equal in length to one wavelength of lightproduced by the source 10. Each half-rotation of the vector of thelinearly polarized wave will generate a cycle of light intensityvariation which is transmitted along a preferential plane ofpolarization by the polarizers 32 and 34 to associated photocells. Thesecycles occur at a relatively rapid rate since a very small movementd)=)./2 of the bed 42, will generate'a cycle. A succession of cyclesestablishes a pattern of moving interference fringes. The photocellsrespond to interference fringe movement. Movement of these fringes isproportional in magnitude and direction to the magnitude and directionof movement of the bed 26. Fringe movement is sensed by photocells 36and 38 to generate electrical outputs e and e When the bed 26 moves tothe left as viewed in FIG. 1, e for example will lead e while oppositemovement causes e to lead, and, e to lag.

The operation of the interferometer may be seen more clearly withreference to the vector diagrams of FIG. 2. In FIG. 2a the direction ofpropagation of the light beams is out of the plane of the paper towardthe reader. The component of light I,,r which is reflected from surface24 of the beam splitter 22 is indicated by the clockwise rotating vectorl r The light component which is reflected by the retroreflector 14 isrepresented by counterclockwise rotating vector l l When the distance D(FIG. 1) between the beam splitter 22 and retroreflector 14 is constant,and since vector l t, and I,r are equal in amplitude and rotate at thesame rate (w), then the resultant of the interfering rotating vectors inFIG. 2a is a vector I which oscillates in amplitude at a constant angle0 with respect to a reference axis X. That is, a stationary objectestablishes a fixed phase relationship 0 between the resultant vectorand the reference axis X at the plane of interference. However, as thedistance D is continuously varied, then the phase relationship 0 at theplane of interference continuously varies. A rotating resultant vectorI, is thereby generated. FIG. 2b indicates the phase relationshipbetween the resultant vector and the X-axis at some other instant oftime during movement of the retroreflector. The polarizers 32 and 34 arepolarized for transmitting the light beam which impinges thereon alongdifferent preselected planes of polarization corresponding to angles 0,and 0, (not shown). For example, the polarizer 32 may be polarized for aplane of transmission corresponding to the Y-axis where 6 +90 while thepolarizer 34 will be polarized for a plane of transmission 0 whichdiffers from 0,. Rotation of the resultant vector past these anglescauses transmission of the interference pattern by the associatedpolarizer. The photocells 36 and 38 will then see" moving fringes oralternate brightening and darkening of the field and the associatedoutput voltages, as indicated hereinbefore, differ in phase inaccordance with the direction of rotation of the resultant vector.

In FIG. 5, an arrangement is illustrated which combines orthogonallyrelated elliptically polarized light beams. The ar' rangement of FIG. 5includes elements which perform functions similar to functions performedby elements of FIG. I and these elements bear the same referencenumerals in FIG. 5. Surfaces 24 and 28 of beam splitter 22 and 27respectively comprise surfaces having conventional dielectric coatingspositioned thereon. These surfaces are oblique with respect to incidentlight beams and exhibit a preference for transmitting components in afirst direction and for reflecting components in a second orthogonaldirection. The optical characteristics of the surface 24 and 28 eachexhibit the following relationships: T, R, and T, R That is, the surface24 transmits a secondary (extraordinary) wave equal in magnitude to themagnitude of a reflected principal (ordinary) wave R, and transmits aprincipal (ordinary) wave T equal in magnitude of a reflected secondary(extraordinary) wave R,.

A circularly polarized input beam to the splitter 22 is divided into aelliptically polarized transmitted beam t and a reflected orthogonallyrelated elliptically polarized beam I n having a vector with an oppositesense of rotation with respect to l t The direction of rotation of thereflected beam l,,!, is reversed by the reflector 14 and theorthogonally related elliptically polarized beams l r, and l t, arecombined at the surface 28. The resultant wave I is generallyelliptically polarized. However, when the reference and reflected beamsare equal, the locus of the resultant on Poincare's sphere is a tippedgreat circle which interests the circle of linear polarization at twopoints. Since the combined beams are orthogonally related, 'thiselliptically polarized resultant wave can be represented as a circle onPoincares sphere. Retarding means amount sufficient to provide rotationof the circle of resultant polarization into coincidence with the circleof linearly polarized light on Poincares sphere. The amount ofretardation provided by these plates depends upon the factor T,, R T,and R, of the surfaces 24 and 28. An output beam from the retardingmeans comprises a linearly polarized light beam having an angle orrotation which varies in accordance with displacement of the object 26.The means for detecting and indicating this rotation and correspondingdisplacement of the object were described with respect to FIG. 1.

FIG. 3 illustrates another arrangement for detecting and indicating thedirection and magnitude of displacement of the object 26. A singlephotocell is utilized in this arrangement and any disadvantageousvariations in relative photocell characteristics of a dual photocellarrangement is thus avoided. Those components of FIG. 3 performingsimilar functions as the components of FIG. 3 performing similarfunctions as the components of FIG. I bear the same reference numerals.The linearly polarized output beam 1,

from the beam splitter 27 and which is produced in the same manner as 1of FIG. 1 is focused on a single photocell 36 by a lens 50. The beam istransmitted successively through an optical chopper or switch 52 whichis excited by an electrical generator 54 and which functions as arotating one quarter wave plate; through a one quarter wave plate 5; andthrough a circular polarizer 56. Optical chopper 52, which is describedand claimed in my copending U.S. Pat. application Ser. No. 566,037'filedon July 18, 1966, and which is assigned to the assignee of the presentinvention comprises an optical quality isotropic material which exhibitsbirefringence when mechanically stressed. The chopper 52 includes apiezoelectric transducer 53, which is electrically excited by thegenerator 54 to cause mechanical vibrations in the optical material.

A combined effect of the chopper 52 and quarter wave plate 55 onthelinearly polarized beam I is the generation of an output vector 1" whichrotates at a frequency f when the object 26 and phase angle 0 areconstant. This rotating vector is transmitted preferentially by thepolarizer 56 when the angle of the vector 1" corresponds to the planeofpolarization of the polarizer 56. As the object 26 is displaced and thephase angle 0 of the vector I varies, the frequency of rotation of thevector I",,, also varies and at a rate in accordance with the magnitudeof object displacement. This frequency variation will be an increase ordecrease corresponding to the direction of object displacement. Anoutput signal from the photocell of FIG. 3 is a frequency modulatedsignal. For example, the generator 54 excites the chopper 52 at somefrequency f As the object 26 is being displaced at a constant rate inone direction, the output frequency of the photocell is, for example, f,+f while a displacement in the opposite direction at the same constantrate generates a photocell signal having a frequency f, f The photocellsignal is applied to the counter 44 through the amplifier and clipper 40while the output of generator 54 is also applied to the counter 44.Counter 44 provides a resultant count indicative of the objectsdisplacement. The direction of displacement is indicated by an increaseor decrease in the initially accumulated count.

It is desirable at times to detect the velocity of displacement of theobject 26. The detector arrangement of FIG. 3 provides an output signalfrom the photocell 36, in response to motion of the body 26, having afrequency which is a linear function f. recorder, or control device.

In a particular arrangement which is not considered to be limiting inany respect, the following components were employed in an interferometerin accordance with FIG. 1.

Retroreflector l4 Polarizer 32, 34

Photocells 36, 38

Perkin-Elmer 02367 Polaroid HN 32 Texas Instruments LS-400 While I haveillustrated and described particular embodiments of my invention, itwill be understood that various modifications may be made thereinwithout departing from the spirit of the invention and the scope of theappended claims.

Iclaim:

1. An interferometer for detecting and indicating motion of an objectcomprising:

light reflector means coupled to the object in a manner for movementtherewith upon displacement of the object; means for causing a firstlight beam having a predetermined polarization to impinge upon saidreflector means;

said reflector means adapted for reflecting the impinging light beam;means for causing a second light beam having a same predeterminedpolarization and orthogonally related with respect to the reflected beamto combine with the reflected beam and to provide a resultant linearlypolarized light beamhaving a plane of polarization rotatable upondisplacement of the object; and, 1

means for operating on said uniformly polarized light beam to provide anelectrical output signal having a frequency modulation componentvaryingin accordance with the velocity of displacement of the object andfor detecting said modulation component to provide an indication of thevelocity of displacement of said object.

2. The interferometer of claim 1 wherein said transmission meansincludes reflective surface means mounted on said object and positionedalong said path of variable length.

3. In an interferometer for measuring distance:

a light reflector means coupled to a movable object for movementtherewith, said reflector means having reflec tive surfaces adapted forreversing the direction of propagation of a polarized impinging lightbeam along a path differing from an incident beam path and for reversingthe direction of polarization thereof; a light beam splitter adapted fordividing an impinging light beam into transmitted and reflectedcomponents; light source means including a laser light source arrangedfor causing polarized light which is polarized in a first direction toimpinge upon said beam splitter; a light beam combination means; saidbeam splitter positioned with respect to said reflector means and withrespect to said light beam combination ,means for causing a transmittedlight component to impinge upon said reflector means and for directing areflected component from said first beam splitter to said light beamcombination means whereby the transmitted I and reflected beams are ofsubstantially equal intensity;

said light beam combination means positioned with respect to said firstbeam splitter and with respect to said reflector means for causing acombination of a reflected light component from said first beam splitterand a reflected light component from said reflector means and forproviding a resultant linearly polarized light beam having a plane ofpolarization rotatable in accordance with motion of the object;

means disposed in the path of said linearly polarized light beam andadapted for transmitting said light beam at first and secondpredetermined planes of polarization; and,

4. The interferometer of claim 3 wherein said light beam combinationmeans comprises a second beam splitter and an optical retarding means.

1. An interferometer for detecting and indicating motion of an objectcomprising: light reflector means coupled to the object in a manner formovement therewith upon displacement of the object; means for causing afirst light beam having a predetermined polarization to impinge uponsaid reflector means; said reflector means adapted for reflecting theimpinging light beam; means for causing a second light beam having asame predetermined polarization and orthogonally related with respect tothe reflected beam to combine with the reflected beam and to provide aresultant linearly polarized light beam having a plane of polarizationrotatable upon displacement of the object; and, means for operating onsaid uniformly polarized light beam to provide an electrical outputsignal having a frequency modulation component varying in accordancewith the velocity of displacement of the object and for detecting saidmodulation component to provide an indication of the velocity ofdisplacement of said object.
 2. The interferometer of claim 1 whereinsaid transmission means includes reflective surface means mounted onsaid object and positioned along said path of variable length.
 3. In aninterferometer for measuring distance: a light reflector means coupledto a movable object for movement therewith, said reflector means havingreflective surfaces adapted for reversing the direction of propagationof a polarized impinging light beam along a path differing from anincident beam path and for reversing the direction of polarizationthereof; a light beam splitter adapted for dividing an impinging lightbeam into transmitted and reflected components; light source meansincluding a laser light source arranged for causing polarized lightwhich is polarized in a first direction to impinge upon said beamsplitter; a light beam combination means; said beam splitter positionedwith respect to said reflector means and with respect to said light beamcombination means for causing a transmitted light component to impingeupon said reflector means and for directing a reflected component fromsaid first beam splitter to said light beam combination means wherebythe transmitted and reflected beams are of substantially equalintensity; said light beam combination means positioned with respect tosaid first beam splitter and with respect to said reflector means forcausing a combination of a reflected light component from said firstbeam splitter and a reflected light component from said reflector meansand for providing a resultant linearly polarized light beaM having aplane of polarization rotatable in accordance with motion of the object;means disposed in the path of said linearly polarized light beam andadapted for transmitting said light beam at first and secondpredetermined planes of polarization; and, means positioned with respectto said latter means for generating first and second electrical signalsrepresentative of the magnitude and direction of motion of theinterference fringes.
 4. The interferometer of claim 3 wherein saidlight beam combination means comprises a second beam splitter and anoptical retarding means.